1. Field of the Invention
This invention relates to a developing apparatus used in an image forming apparatus such as a copier or a printer using the electrophotographic method or the electrostatic recording method to develop an electrostatic image on an image bearing member.
2. Related Background Art
In recent years, higher speeds, higher functions and coloring have been advanced in image forming apparatuses such as electrophotographic apparatuses, and printers of various types have become available in the market.
From the viewpoint of the higher speed of printers, the studies and development of in-line type apparatuses in which a plurality of electrophotographic units for forming different color images are disposed in series and these are driven at a time to thereby effect image formation have been advanced, and these apparatuses have wide possibilities in business use because they can form color images at a high speed.
Particularly, because of the small number of process constituents and accordingly the ease of downsizing and cost reduction, there have been developed numerous in-line printers of the transferring belt type in which paper is adsorbed to a transferring belt serving also as a conveying belt and the superimposition transfer of toner images is effected from a plurality of image forming units.
Further, in recent years, there has been developed a longitudinal pass printer of the in-line type in which process forming units are disposed longitudinally with a view to reduce the installation area thereof. In this type, it is necessary to convey paper upwardly against gravity and therefore, bad adsorption leads to the jam of the paper and thus, importance is particularly attached to the adsorbing performance.
Generally, as shown in FIG. 2 of the accompanying drawings, paper passes between adsorbing rollers which are disposed in opposed relationship with the upstream entrance of a transferring belt and to which a voltage has been applied, whereby charges are imparted to the paper, which is thus electrostatically attracted to and conveyed by the transferring belt.
The adsorbing roller is normally urged against the transferring belt generally in the most upstream portion of the whole station, and assumes a construction in which it is driven to rotate relative to the transferring belt.
Regarding the printer of the in-line type, the problem that it is inferior in usability because of the fact that in the construction thereof, the main body thereof is liable to become bulky and has many interchangeable parts has been pointed out.
Particularly regarding the interchangeable parts, it is necessary to frequently interchange cartridges for four colors and a waste toner container for the transferring belt and in addition, considering the life of the main body including a fixing device, a sheet feeding roller, etc., the user must interchange many parts, and this becomes a factor which increases the user""s running cost and further, the labor required for the interchange and the obtainment of the parts become the user""s load and therefore, it is required to decrease the interchangeable parts as much as possible.
The waste toner container for the transferring belt which is one of the interchangeable parts is for collecting therein toners appeared when cleaning is effected so that toners left on the transferring belt by the image formation on the transferring belt for the detection of jam and density and the detection of registration may not adversely affect the next image printing.
Generally the cleaning of the transferring belt is to scrape off the toners by a cleaning blade or a rotary fur brush brought into pressure contact with the surface of the transferring belt and collect them in the waste toner container.
This waste toner container is not essentially necessary for printing, but is provided inevitably and the interchange thereof is virtually meritless to the user and therefore has remarkably spoiled usability.
Also, the provision of the interchangeable waste toner container has led to the problem of the bulkiness and complication of the construction of the main body.
As a different way of thinking, there is the concept of making the waste toner container large in size and install it on a transferring belt unit or the main body in order to improve the convenience in usability, but when the waste toner container is filled up due to some cause or the other, it is necessary to bodily interchange the unit, and this has likewise become a factor for the bulkiness or risk of the main body.
As a method of solving these problems, there has been proposed the technique of not providing a cleaner for the transferring belt, but electrostatically collecting the toners on a photosensitive body. Particularly in the printer of the in-line type, there are four process cartridges and therefore, there are four chances for collecting the toners on the belt per one round of the transferring belt, and this is advantageous.
Further, during the cleaning sequence for the transferring belt, the polarities of voltages applied to the transferring member at four stations are made different and an electric field in the opposite direction is formed at each station to thereby collect toners of the positive and negative polarities.
Furthermore, it is also possible to adopt such a construction in which the voltage applied is made great to thereby charge the toners on the transferring belt in the transferring portion and collect the toners at the next station.
When these techniques are used, it is necessary to provide a transferring belt cleaning sequence for changing over the voltage of the transferring portion, or incorporate a cleaning sequence in the post-rotation subsequent to image formation, discretely from the ordinary image forming sequence.
On the other hand, in the image forming apparatus of the in-line type, respective color images are formed by image forming units and therefore, such image forming apparatus essentially suffers from the disadvantage that the color balance is liable to be destroyed or it is difficult for the registration of each color.
Heretofore, regarding the color balance in each unit, use has been made of means for forming a density patch image of each color in an intermediate transferring member (hereinafter referred to as the ITB) or an electrostatic transferring belt, reading it by a density detecting sensor, and feeding it back to a process forming condition such as a high voltage condition or laser power to thereby adjust the maximum density of each color and a halftone gradation characteristic.
Also, likewise in color registration, use is made of means for forming a patch for registration detection on a transferring belt, reading it by an optical sensor, and feeding it back to an image writing position or the like to thereby effect correction.
Generally, the density detecting sensor irradiates the density patch with a light source, detects the intensity of reflected light by a light receiving sensor, handles the image density as the intensity information of the light, and electrically processes it.
Image density control has as its objects to keep the maximum density (hereinafter referred to as the Dmax) of each color constant, and to keep the halftone gradation characteristic linear relative to an image signal.
The control of the Dmax is very meaningful in keeping the color balance of each color constant and at the same time, preventing the scattering or bad fixing of color-superimposed characters by the excessive deposition of toners.
On the other hand, halftone gradation control is usually to effect such image processing that in order to prevent output density from deviating relative to input and output image signals by a non-linear input-output characteristic (xcex3 characteristic) peculiar to electrophotography to thereby fail to form a natural image, the xcex3 characteristic is negated and the input-output characteristic is kept linear.
On the other hand, a registration sensor reads a registration patch formed by a line image by a focused light receiving sensor, and electrically processes a variation with time in the intensity of the signal of the light receiving sensor when the registration patch passes it as positional deviation information.
When effecting such optical detection, it is effective to once measure the reflectance of the transferring belt forming the toner thereon as the ground before reading the optical information of the toner image, and compare this measured value with the measured value after the formation of the toner to thereby improve detection accuracy.
Specifically, by normalizing the sensor output after the toner image has been formed, with the sensor output corresponding to the reflectance when the ground of the transferring belt has been measured as 1, it becomes possible to always measure the density of a stable toner image irrespective of the toner stain of the surface of the transferring belt, the roughness of the surface of the transferring belt by the endurance thereof, etc.
However, when an apparatus of a type having no cleaning station for the transferring belt is constructed with an aim at improved usability, the unintended toner development from a developing device and fog cause problems.
In a case where the contact developing system is u s ed as the developing system, when the developing device is started (rotated) with a developer carrying member and an image bearing member being in contact with each other, the toner in the first contact nip portion is used for development to thereby create a lateral line-line fog image.
This phenomenon is seen more remarkably when the apparatus is stopped for a long period and the triboelectricity of the developer is reduced, and there arises the problem that this toner used for development shifts to the transferring belt to thereby stain the back of the paper during printing.
Also, when during the prerotation before printing, the developing device is rotating while being in contact with the image bearing member, fog toner used for development adheres, though slightly, to the transferring belt. If a cleaning apparatus is provided for the transferring belt, belt cleaning is done once per one round of the transferring belt and therefore, no toner accumulates on the transferring belt, but in a cleaningless system, the transferring belt has sometimes been stained by fog toner due to long-time idle rotation such as prerotation to thereby cause the stain of the back of the next image.
Also, the presence of such toner stain and fog on the transferring belt might adversely affect the ground measurement when the optical detection of the toner density or the like such as density detection or registration detection is effected, thereby causing a detection error.
It is an object of the present invention to provide an image forming apparatus which can prevent the fog in the contact development at the start of image formation.
It is another object of the present invention to provide an image forming apparatus which can reduce the toner stain of a transferring belt.
It is still another object of the present invention to provide an image forming apparatus comprising:
an image bearing member;
a charger for charging the image bearing member; image forming means for selectively erasing a charge on the image bearing member to thereby form an electrostatic image;
a developing device for developing the electrostatic image, the developing device having a developing roll contacting with the image bearing member and having a developing bias applied thereto; and
contacting and spacing means for contacting and spacing the developing roll with respect to the image bearing member;
the contacting and spacing means contacting the developing roll having the developing bias applied to the area thereof charged by the charger with the image bearing member at a start of image formation.
Other objects of the present invention will become apparent from the following description.